On what grounds could I justify a refund of the deposit?
Who are you buying it from? Seems bizarre for a deposit to non-refundable.
If the seller isn't willing to refund your deposit, I think that speaks for itself.
The first Leon I looked was as cleeeeean as a whistle - 16k miles on a 4yr old car, it threw an EML and limp mode on a test drive, and I put a deposit on to secure it while they got it sorted. Long story short, it all seemed so good to be true, and then discussing with a few on here (a bit like you did on your other thread) I decided against it, and despite the seller spending apparently a grand on sorting it out, I got my deposit back and walked away.
The car I eventually bought needed a £200 deposit to secure before I even saw it (it was 200miles away), and that was also unconditionally refundable up until the point I paid the balance.
You can mitigate all these issues and dodgy history as much as you want and its your money after all, but you are asking for folks advice and they are giving it.
There's every chance it could all work out lovely, there is a chance you could walk, go and buy a minter and it goes wrong as you drive away - that's just the fun of buying a car.
I hope whatever you choose to do works out for you though
